1. Field of the Invention
This invention relates to thin-film magnetic recording media of excellent high-density recording characteristics, a method for their manufacture, and the fabrication equipment thereof.
2. Description of the Prior Art
Magnetic recording and playback systems with increasingly high-density recording characteristics have come into being. This has created an increasing demand for magnetic recording media with superior short-wave read/write characteristics. The majority of the magnetic recording media currently in use are coated magnetic recording media which are made by applying a coating of magnetic powder to a substrate. Although improvements have been made to satisfy the demand, efforts to achieve continued improvements have encountered certain limits.
To overcome the limits, thin-film magnetic recording media have been developed. These recording media, which are produced by the vacuum vapor deposition method, the sputtering method, or the plating method, offer excellent short-wave read/write characteristics. For thin-film magnetic recording media, the use of the following magnetic layers is being considered: Co, Co--Ni, Co--Ni--P, Co--O, Co--Ni--O, Co--Fe--O, Co--Ni--Fe--O, Co--Cr, and Co--Ni--Cr.
From the point of view of the suitability of application to magnetic tape, Co--O and Co--Ni--O films, which are partially oxidized films, are considered to be the best. A vapor deposition tape in which Co--Ni--O is used as a magnetic layer has already been commercialized as a Hi8 VCR tape (ME tape). From a production efficiency standpoint, the vacuum vapor deposition method with oblique incidence is used as a method for producing thin-film magnetic recording media.
A detailed explanation of one example of a vapor deposition tape fabrication method follows, with reference to FIG. 12. FIG. 12 shows an example of the internal configuration of the continuous vacuum vapor deposition apparatus for producing vapor deposition tapes by oblique incidence.
Substrate 100, composed of a polymer material, runs in the direction of arrow R along a cylindrical drum 200. Reference numerals 201 and 202, respectively, denote a supply roll and a takeup roll for substrate 100. Reference numeral 203 denotes a free roller, several of which are provided at appropriate locations to ensure that substrate 100 runs evenly. In some cases cylindrical drum 200 is cooled by some cooling media, such as cooling water, in order to prevent thermal damage to the substrate from radiation heat from the evaporation source during the vapor deposition process or from the heat of condensation that occurs when evaporated atoms deposit on the substrate.
The deposition of evaporated atoms, vaporized from evaporation source 120, onto substrate 100 forms a magnetic layer. An electron beam evaporation source is well suited to be evaporation source 120. In this source, an alloy based on cobalt is filled as evaporation substance 130. The reason for the use of an electron beam evaporation source as an evaporation source is to vaporize high-melting-point metals, such as cobalt, at a high rate of evaporation. In the figure, electron beam 110 is depicted schematically in terms of an arrow.
Reference numerals 150 and 151 denote shielding plates that are provided in order to prevent the deposition of superfluous evaporated atoms onto the substrate and to define the range in which evaporated atoms strike substrate 100. A magnetic layer is formed when the evaporated atoms, passing through opening 180 composed of the shielding plate, reach the substrate. The incident angle of evaporated atoms is defined as the angle formed by an incident direction of evaporated atoms and a line normal to substrate 100. Shielding plate 151 defines the initial incident angle .phi.i. Likewise, shielding plate 150 defines the final incident angle .phi.f. It should be noted that the initial incident angle .phi.i in the fabrication of Hi8 VCR tape ME is approximately 90.degree., and the final incident angle .phi.f is approximately 30.degree.. .phi.i is 90.degree. when evaporated atoms are in contact with substrate 100, in which case shielding plate 151 can be omitted.
An oxygen supply nozzle 170 is provided at the edge of shielding plate 150 in order to introduce oxygen into the vacuum tank during vapor deposition. By optimizing the amount of oxygen introduced, vapor deposition tapes of excellent read/write characteristics and practical utility can be obtained.
The magnetic layer of the magnetic recording medium thus fabricated has a columnar structure, and its easy axis is inclined relative to the line normal to the magnetic layer. In other words, the easy axis is neither in the film nor in the direction of the line normal to the film surface. Rather, it is in a direction that is slanted with respect to the normal line on the normal surface, which includes the incident direction of evaporated atoms with respect to the substrate. For example, in commercial Hi8 VCR ME tapes, the easy axis is inclined approximately 20.degree. on the normal surface that includes the lengthwise direction of the tape. Here, the lengthwise direction of a tape is the direction along the length of the tape. In the fabrication equipment shown in FIG. 12, this direction is the direction in which substrate 100 runs. The magnetization that is recorded by a ring-type magnetic head remains in the direction of the obliquely slanted easy axis and forms a magnetization mode different from conventional longitudinal recording.
The formation of such a slanted magnetization mode produces a significant improvement in high-density recording characteristics over conventional longitudinal recording media.
Further, to improve the read/write characteristics and practical utilization characteristics, a double-layer structure magnetic layer has been proposed (Japanese Patent laid-Open Publication H3-54719). As noted above, when a magnetic layer is formed by the oblique vapor deposition method relative to a running substrate, the incident angle varies from an initial incident angle to a final incident angle between the spot where the film formation process is begun and the spot where the film formation process is terminated. As a result, the columnar crystal particles that compose the film are inclined and bend relative to the substrate surface. In a double-structure magnetic layer, the performance characteristics of the magnetic layer can be varied according to the angle of inclination of the columnar crystal particles that compose the magnetic layers.
For example, if the angle of inclination of the columnar crystal particles for a layer is opposite to the direction in which the substrate runs, during read/write, the change in playback output due to a relative moving direction between the ring-type magnetic head and the magnetic layer tends to decrease. Further, it has been proposed to make the film thickness of the lower magnetic layer greater than that of the top magnetic layer in order to reduce the change in playback output (Japanese Patent laid-Open Publication H3-78028).
If the direction of inclination of the columnar crystal particles in the different layers is the same as the running direction of the substrate, the change, during read/write, in playback output due to a relative moving direction between the ring-type magnetic head and the magnetic layer tends to increase. However, in a certain direction, high playback output can be obtained. Further, a proposal has been made to make the oxygen content of the top magnetic layer greater than that of the bottom magnetic layer in order to achieve a better head touch and to improve the tape's durability (Japanese Patent laid-Open Publication S62-236122). Both approaches offer the advantage of decreasing the noise level, as compared to a single-layer magnetic layer, by effecting a double-layer structure.
In both the single- and double-layer structures, the film thickness of the magnetic layer is approximately 0.2 .mu.m.